Sorry for my mistake my PC is Latitude E6320 with CPU i5-2520m so Sandy Bridge, no ?
After following this guide :
- http://www.insanelymac.com/forum/topic/315493-guide-run-macos-sierra-on-a-dell-latitude-e6320/
Everything works fine on Sierra 10.12
It's when I want to upgrade to High Sierra 10.13 that everything is mess up.